Abstract: Tamoxifen reduces primary breast cancer incidence, yet causes serious side effects. To date, few women with increased breast cancer risk have elected to use tamoxifen for chemoprevention. The objective of the study was to determine women's knowledge of and attitudes toward tamoxifen following exposure to a tailored decision aid (DA). A total of 632 women with a 5-year risk of breast cancer > or = 1.66% (Mean = 2.56, range = 1.7-17.3) were recruited from two healthcare organizations. Participants viewed an online DA that informed them about their 5-year risk of breast cancer and presented individually tailored content depicting the risks/benefits of tamoxifen prophylaxis. Outcome measures included behavioral intentions (to seek additional information about tamoxifen, to talk to a physician about tamoxifen, and to take tamoxifen); knowledge; and perceived risks and benefits of tamoxifen. After viewing the DA, 29% of participants said they intended to seek more information or talk to their doctor about tamoxifen, and only 6% believed they would take tamoxifen. Knowledge was considerable, with 63% of women answering at least 5 of 6 knowledge questions correctly. Participants were concerned about the risks of tamoxifen, and many believed that the benefits of tamoxifen did not outweigh the risks. This study is the largest to date to test women's preferences for taking tamoxifen and one of the largest to have tested the impact of a tailored DA. After viewing the DA, women demonstrated good understanding of tamoxifen's risks and benefits, but most were not interested in taking tamoxifen for breast cancer chemoprevention.